Wireless local area networks (WLANs) are gaining in popularity, and the demand for WLAN bandwidth is growing. The original WLAN standards, such as IEEE 802.11, were designed to enable communications at 1-2 Mbps in a band around 2.4 GHz. More recently, IEEE working groups have defined the 802.11a, 802.11b and 802.11g extensions to the original standard, in order to enable higher data rates. The 802.11a standard, for example, provides data rates up to 54 Mbps over short distances in a 5 GHz band using a multi-tone, orthogonal frequency domain multiplexing (OFDM) scheme. The 802.11b standard defines data rates up to 11 Mbps using a single-carrier modulation scheme in the 2.4 GHz band. The newer 802.11g standard permits data rates up to 54 Mbps in the 2.4 GHz band using OFDM, and is also backward-compatible with legacy 802.11b products. In the context of the present patent application and in the claims, the term “802.11” is used to refer collectively to the original IEEE 802.11 standard and all its variants and extensions, unless specifically noted otherwise.
In 802.11 WLANs, a fixed access point communicates on a predefined frequency channel with wireless mobile stations (also referred to simply as “stations”) in its vicinity. The group of stations communicating with the same access point is referred to as a basic service set (BSS). Generally, with the exception of certain broadcast messages, the access point can transmit downlink signals only to one station at a time. Otherwise, the stations would receive multiple, interfering signals, which they would then be unable to decode. Similarly, to prevent simultaneous uplink transmissions, the 802.11 standard provides a contention avoidance mechanism, which requires a station to refrain from transmitting when it senses other transmissions on its frequency channel.